I said I knew a very wise man so much of Sir Christopher's sentiment, that he believed if a man were permitted to make all the ballads he need not care who should make the laws of a nation, and we find that most of the ancient legislators thought that they could not well reform the manners of any city without the help of a lyric, and sometimes of a dramatic poet.
Source, in full
An ACCOUNT of A CONVERSATION concerning A RIGHT REGULATION of GOVERNMENTS For the common Good of Mankind: In A LETTER to the Marquiss of Montrose , the Earls of Rothes, Roxburg and Haddington , From London the first of December, 1703'. Later variants express the sentiment in the first person, e.g.
Notes from Wikiquote
Let me make the songs of a nation, and I care not who makes its laws. | Give me the making of a people's songs, and I care not who makes its laws. | They may also substitute equivalent words, such as "songs" for "ballads" or "country" for "nation". The sentiment is sometimes attributed to Plato, but does not appear in his works. Austin Matzko has discovered that the mistaken attribution probably originated in an ambiguous sentence in Donald J. Grout's A History of Western Music (1973, p. 8).